A centimetre-wave excess over free-free emission in planetary nebulae
Journal article, 2007
We report a centimetre-wave (cm-wave, 5-31 GHz) excess over free-free emission in planetary nebulae (PNe). Accurate 31- and 250-GHz measurements show that the 31-GHz flux densities in our sample are systematically higher than the level of optically thin free-free continuum extrapolated from 250-GHz. The 31-GHz excess is observed, within one standard deviation, in all 18 PNe with reliable 31- and 250-GHz data, and is significant in nine PNe. The only exception is the peculiar object M2-9, whose radio spectrum is that of an optically thick stellar wind. On average the fraction of non-free-free emission represents 51 per cent of the total flux density at 31 GHz, with a scatter of 11 per cent. The average 31-250 GHz spectral index of our sample is = -0.43 +/- 0.03 (in flux density, with a scatter of 0.14). The 31-250 GHz drop is reminiscent of the anomalous foreground observed in the diffuse interstellar medium (ISM) by cosmic microwave background (CMB) anisotropy experiments. The 5-31 GHz spectral indices are consistent with both flat spectra and spinning dust emissivities, given the 10 per cent calibration uncertainty of the comparison 5-GHz data. However, a detailed study of the objects with the largest cm-excess, including the low-frequency data available in the literature, shows that present spinning dust models alone cannot explain the cm-wave excess in PNe. Although we have no definitive interpretation of our data, the least implausible explanation involves a synchrotron component absorbed by a cold nebular screen. We give flux densities for 37 objects at 31 GHz, and for 26 objects at 250 GHz.
INTERSTELLAR-MEDIUM
RADIO-SYNTHESIS OBSERVATIONS
ABSORPTION-COEFFICIENTS
continuum : ISM
METALLIC NEEDLES
DUST GRAINS
GAUNT FACTORS
MAGNETIC-FIELDS
CENTRAL STARS
radio
submillimetre
CONTINUUM
planetary nebulae : general
COSMIC BACKGROUND IMAGER
OBSERVATIONS
radiation mechanisms : general
Author
S. Casassus
University of Chile (UCH)
Lars-Åke Nyman
Chalmers, Department of Radio and Space Science, National Facility for Radio Astronomy
C. Dickinson
Jet Propulsion Laboratory, California Institute of Technology
California Institute of Technology (Caltech)
T. J. Pearson
California Institute of Technology (Caltech)
Monthly Notices of the Royal Astronomical Society
0035-8711 (ISSN) 1365-2966 (eISSN)
Vol. 382 4 1607-1622Subject Categories (SSIF 2011)
Astronomy, Astrophysics and Cosmology
DOI
10.1111/j.1365-2966.2007.12366.x